1. Field of the Invention
The invention relates to a circuit for the surveillance of the switching state of a power transistor, in particular for energizing a brake system.
2. Brief Description of the Background of the Invention Including Prior Art
So-called "intelligent" power transistors have become known in recent years which are able to protect themselves by way of integrated sensors and integrated electronic circuits against short circuits, over-voltages, and excess temperature, cf. Journal Siemens Components 25, (1987) Issue 5.
These transistors comprise additionally a surveillance arrangement integrated into the casing. The surveillance arrangement switches the transistor off in case of the presence of impermissible operating conditions. Based on this, the power transistor is protected against destruction.
It is further possible to inform a supervising microcomputer if a short circuit or an interruption of the load is present.
However, an incomplete short circuit or, respectively, a so-called shunt circuit, is not being recognized, i.e. cases where the resistance is too small compared with to the load resistance of the load device controlled. This occurs, for example, in such cases where a load is connected with a real short circuit via a long connection line. Such an error generates an excessive current, which results after some time in over-heating of the transistor and/or of the load.
In addition to the load, the final power transistor for energizing the load can also be defective. This is the case if the final power transistor is switched to a controlled blocked state, but nevertheless allows the passage of an excessive so-called leakage current. It is imperative that this error also be indicated if the leakage current reaches and surpasses impermissibly high values in order to allow to timely undertake appropriate countermeasures.